Among various types of charged particle devices, the most widely used one is an electron microscope. The electron microscope has an electron gun chamber the inside of which is kept at a low pressure and the electron gun chamber houses a cathode as an electron source for emission of electrons, an extraction electrode for extraction of electrons from the cathode, and an acceleration electrode for acceleration of extracted electrons. The accelerated electrons constitute an electron beam which is irradiated on a specimen to observe the specimen. In such a field-emission type electron microscope, a high electric field is applied between the cathode and, the extraction electrode and acceleration electrode, in order to produce an electron beam, as described in JP-A No. 58(1983)-161234 (FIG. 2).